For easy driving of a vehicle and for reducing fatigue to the driver, various kinds of power transmission devices for vehicles have, in recent years, been widely used for easy driving. A representative example will be a so-called automatic transmission (AT) combining a torque converter and a planetary gear device together. A power transmission device which uses a transmission of the type of parallel shaft gear mechanism similar to the so-called manual transmission in combination with an automatic clutch, is one of such automatic power transmission devices for vehicles. In this power transmission device, a clutch disposed between an engine and a transmission is provided with a clutch actuator, and the clutch is automatically connected or disconnected at the time when the driver changes the speed by shifting the gear by using a shift lever or at the start of the vehicle eliminating the need of operating the clutch pedal by the driver. There has been further proposed a power transmission device which automatically shifts the gear depending upon the traveling condition of the vehicle by using an electronically controlled device obviating the need of operating the shift lever by the driver.
A clutch (dry type single disk clutch) installed between an engine and a transmission is provided with a clutch disk 101 which has a friction disk fixed to the peripheral portion thereof as shown in FIG. 6. The clutch is slidably fitted by spline to a transmission input shaft 103 rotatably supported by a crankshaft 102 of the engine. A pressure plate 105 is provided on the back of the friction disk of the clutch disk 101 to bring the friction disk into pressed contact with a flywheel 104 at the rear end of the crankshaft 102. Further, a diaphragm spring 107 is attached to a clutch cover 106 that is fixed to the flywheel 104. When the vehicle is normally traveling, the diaphragm spring 107 brings the clutch disk 101 into pressed contact with the flywheel 104 via the pressure plate 105 and, therefore, the engine power is transmitted to the transmission input shaft 103 via the clutch disk 101.
The clutch is equipped with an operation mechanism for connecting and disconnecting the transmission of power, and the operation mechanism is constituted by a release bearing 108, a release fork 109, a clutch actuator 110 and the like. The clutch actuator 110 is a fluid pressure cylinder operated by a pneumatic pressure or a hydraulic pressure, and its piston is coupled to one end of the release fork 109. Provision is, further, made of a stopper 111 for mechanically limiting the movement in order to prevent the occurrence of damage to the clutch actuator 110 and the like caused by excessively large movement of the piston.
At the time of cutting off the engine power for shifting the gear of the transmission, the working fluid is fed to the clutch actuator 110 to displace one end of the release fork 109 toward the right in the drawing. The other end of the release fork 109 displaces toward the opposite direction, causing the release bearing 108 coming in contact therewith to slide leftward so that the diaphragm spring 107 moves as represented by a two-dot chain line in the drawing. Therefore, the spring force that pushes the pressure plate 105 is released, and the transmission of the engine power to the transmission input shaft 103 is cut off. To connect the clutch again after having finished the gear shift, the working fluid in the clutch actuator 110 is discharged, and the release fork 109 is moved leftward by a return spring 112 or the like. The state of clutch connecting (rate of connection) is determined by the movement of the clutch piston.
A clutch control device which automatically connects and disconnects the clutch at the time of gear shifting is provided with a working fluid pressure source such as an air tank that feeds the working fluid, a stroke sensor for detecting the movement of the piston of the clutch actuator, and control valves for controlling the amount of the working fluid in the clutch actuator. The clutch control device executes the clutch control at the time of gear shifting. Usually, the control valves are arranged in the working fluid feed pipe and in the discharge pipe, respectively. The rate of connection of the clutch is controlled by opening and closing these two control valves. There has also been known a clutch control device which feeds and discharges the working fluid in the clutch actuator by using a single flow rate control valve as disclosed in, for example, Japanese Patent No. 3417823.
In the clutch control device that uses a single flow rate control valve as shown in a circuit diagram of FIG. 7, the flow rate control valve 1 is connected to a communication passage 2 communicated with the clutch actuator 110, to a pressure source passage 4 communicated with the working fluid pressure source 3 such as an air tank, and to a discharge passage 5 for discharging the working fluid from the clutch actuator 110 and, further, includes three ports, i.e., a communication port 2p, a pressure source port 4p and a discharge port 5p formed therein and opened to the respective passages. The flow rate control valve 1 of FIG. 7 is a proportional control valve equipped with a drive device of the type of electromagnetic solenoid, and has such flow rate characteristics that the flow rate of the working fluid that flows through varies depending upon the position of a valve body 6. The flow rate control valve 1 is connected to a flow rate control valve control device 9 that sets the position of the valve body 6 by controlling the amount of electric current to a coil 8 responsive to a detection signal from a stroke sensor 7.
As shown in detail in the operation view of FIG. 8, the valve body 6 of the flow rate control valve 1 has two lands on the way thereof, one end of the valve body 6 being coupled to a moving yoke 10 of the electromagnetic solenoid. A spring 11 is arranged at the other end of the valve body 6, and the position of the valve body 1 is determined by a balance between the magnetic force acting on the moving yoke 10 and the resilient force of the spring 11. When the flow of current to the coil 8 is interrupted (amount of current, 0%), the valve body 6 is pushed by the spring 11 and assumes a position shown in FIG. 8(b) whereby the communication port 2p communicates with the discharge port 5p, and the working fluid in the clutch actuator 110 is discharged to the exterior permitting the clutch to be connected. If the electric current flowing into the coil 8 assumes a maximum value (100%), the valve body 6 is brought to a position shown in FIG. 8(c) compressing the spring 11, and the communication port 2p communicates with the pressure source port 4p. Therefore, the working fluid in the pressure source 3 is introduced into the clutch actuator 110 through the communication port 2p, and the clutch is disconnected. When a 50%-current flows into the coil 8, the valve body 6 is brought to a position of FIG. 8(a), i.e., brought to the neutral position, and the communication port 2p is cut off from the power source port 4p and the discharge port 5p; i.e., the stroke of the clutch is maintained at this position. The flow rate characteristics of the flow rate control valve 1, i.e., the flow rates of the working fluid responsive to the electric current flowing into the coil 8 are as represented by a solid line in FIG. 9.
At the time of gear shifting, the clutch must be disconnected and connected quickly without causing shift shock. Therefore, at the time of connecting the clutch after having shifted the gear (after the gears are engaged), the piston of the clutch actuator 110 is, first, quickly moved in a direction of connection so as to quickly pass through an invalid region where the torque is not substantially transmitted, and the rate of connection is gradually increased in the so-called half-engage clutch region where the torque starts transmitting in order to avoid the shift shock caused by a sharp increase in the rate of connection as illustrated in a graph of FIG. 10 that shows changes in the stroke. To execute the above control, the stroke sensor 7 detects the displacement of the clutch actuator 110, and the flow rate control valve control device 9 controls the flow rate control valve 1 responsive thereto to vary the amount of the working fluid in the clutch actuator 110 so as to attain a target stroke.    Patent document 1: Japanese Patent No. 3417823